Collier, KJ. & Winterbourn, MJ. (1990). Structure of epilithon in some acidic and circumneutral streams in South Westland, New Zealand. New Zealand Natural Sciences 17:1-11. The composition and biomass of epilithon colonising introduced substrata was investigated in two acid (pH 4.3-5.7), brownwater and two circumneutral (pH 6.6-8.0), Clearwater streams in South Westland. Scanning electron microscopy showed that epilithon in the circumneutral streams was composed primarily of diatoms (mostly

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... phonema subclavatum and Achnanthes minutissima) and blue-green algae, whereas in the acid streams other species of diatoms (mainly Fragilaria and Eunotia species), filamentous algae (including Stigeoclonium sp. and Tribonema sp.) and deposits of amorphous material (possibly adsorbed or precipitated organic carbon) were common. Energy-dispersive X-ray analysis indicated that silicon was the most abundant element in epilithon at all sites, presumably reflecting the presence of siliceous diatom frustules or mineral particles. Calcium was significantly more abundant (4-30 times) in epilithon from the circumneutral streams, whereas iron was significantly more common in epilithon at one of the acid, brownwater sites. The ratios of organic carbon to chlorophyll a + phaeophytin a concentration were significantly greater at acid than circumneutral sites (x = 452 and 197, respectively), indicating that algae comprised proportionately less of the epilithic carbon at the former sites. Adsorbed or precipitated organic carbon appears to be a major component of epilithon in acid, brownwater streams, whereas low algal biomass may be maintained by a combination of nutrient-limitation, growth inhibition by humic substances, and physical abrasion of stone surfaces. KEYWORDS: epilithon-scanning electron microscopy-energy-dispersive X-ray analysis-acid streams-New Zealand.